DJGPP and the Allegro game library were my first foray into C++ in around 1996-1997 if I recall. Having access to a proper 32-bit protected mode system in DOS (without having to pirate a bunch of other compilers!) was a pretty incredible step forward for the community.

I pretty quickly moved onto building in Win95/98 with MSVC not long after, but I am very grateful to DJ for the compiler and Shawn Hargreaves for Allegro.